As is known, an epyciclic transmission comprises a sun gear, a ring gear and a plurality of toothed planet wheels, which are interposed between the sun gear and the ring gear and are supported by a carrier. A transmission of such a type is capable of transmitting the motion between coaxial shafts rotating at different speeds, and is very effective in providing such a function while maintaining small weight and volumes. Epyciclic transmissions are largely used also in aeronautical engines, for example to transmit the motion to the fan in turbofan engines.
In most applications, the carrier is of static type and is coupled to the fixed structure of the engine by means of a flexible element. Under these conditions, the components supported by the carrier (gears of the planet wheels, possible bearings, etc.) are lubricated without particular difficulty via ducts which are fixed with respect to the engine structure and to the carrier.
Nevertheless, certain applications employ a rotating carrier, for example when the carrier is connected to a rotating (driving or driven) shaft or when there is a need to maintain the same rotation direction between the sun gear and the ring gear. In these cases, a problem occurs when transferring the lubricant oil in an efficient and reliable manner from a static part (typically an oil tank) to a rotating part, i.e. the carrier and the components supported by the same.
The solutions currently in use include one or more pumps which transfer the oil under pressure into an annular recess about a sleeve which is fixed with respect to the carrier. The oil radially enters a passage of the sleeve and from here is conveyed to the components requiring lubrication. The oil pressure in the annular recess is ensured by seals configured to keep a particularly small radial backlash between the static part and the sleeve. The amount of such a backlash is accurately determined in the design step, so as to maximize the volumetric efficiency for the transfer of the lubricant oil.
The above-described oil transfer systems of known type are not satisfactory due to the high accuracy required when machining and mounting the components which define the aforesaid radial backlash. Furthermore, during the assembly operation, there is a non-negligible risk of damaging the seals when the sleeve is axially inserted, whereby the correct sealing is compromised.
Moreover, the above-described oil transfer system of known type is not able to compensate for the wear and inevitable variations of relative position between the static part and the rotating part (due to differences in temperature between the start-up conditions and the running conditions, for example). Furthermore, in current epyciclic transmissions for vehicles, the pressure and flow rate values required for the lubrication and the peripheral speed values of the carrier are relatively high and therefore may compromise the desired sealing, especially when the components are worn.